MathUtil.asinh, acosh and atanh use the textbook log formulas without range reduction: asinh and acosh return Infinity for every finite |x| >= 2^512, asinh and atanh return 0 for tiny nonzero x, and acosh of a large negative x returns an infinity instead of the documented NaN, also through the expression functions asinh, acosh and atanh

Finding LFV-327Area: util / MathUtil inverse hyperbolic functions (asinh, acosh, atanh), reached through the expressions module's default functions asinh, acosh and atanhPriority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-327

AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.

What it shows

You call MathUtil.asinh(1e200), or evaluate the expression asinh(x) for a document whose x is 1e200, and expect about 461.21, because the inverse hyperbolic sine of a finite number is finite (at most about 710.5). Instead you get Infinity, because the code computes log(sqrt(x*x + 1) + x) and x*x overflows for every |x| >= 2^512; acosh does the same, asinh(1e-17) and atanh(1e-17) return 0 instead of 1e-17 because 1 + x*x (or (1+x)/(1-x)) rounds to exactly 1, and acosh(-1e8) returns -Infinity (acosh(-1e200) +Infinity) although its javadoc says that any argument below 1 gives NaN.

The test shows that MathUtil.asinh returns Infinity for the finite argument 1e200, because it computes log(sqrt(a * a + 1) + a) and a * a overflows for every |a| >= 2^512.

Expected vs actual

Expected: asinh(1e200) = ln(2e200) = 461.2101657793691. Actual (-ea and -da): expected:<461.2101657793691> but was:<Infinity>.

Files

• src/org/apache/lucene/util/TestMathUtilAsinhLargeArgument.java (JUnit test)

Run it

• Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
• Tested on Lucene main 645d4fe14a20 (2026-10-28): reproduced.
• Recorded outcome of run 1 on Lucene main 645d4fe14a20: 1 test(s) run, 1 failed.
• How to read the outcome: a failing test is the finding reproducing (the failure message states expected against actual).
• Seeds: a rerun with the same seed replays the same random choices. Assertions off (-da -dsa -Dtests.asserts=false) is the production-like setting.

# Lucene checkout at the tested commit, built with: ./gradlew --no-daemon assemble   (JDK 25)
LUCENE=/path/to/lucene; V=12.0.0-SNAPSHOT
CP=$LUCENE/lucene/core/build/libs/lucene-core-$V.jar
CP=$CP:$LUCENE/lucene/codecs/build/libs/lucene-codecs-$V.jar
CP=$CP:$LUCENE/lucene/test-framework/build/libs/lucene-test-framework-$V.jar
CP=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$CP:junit-platform-console-standalone-6.1.3.jar   # JUnit Platform console launcher (Maven Central)

javac -d classes -cp "$CP" $(find src -name '*.java')

# run 1: assertions on, seed DEADBEEF
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
     -cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.util.TestMathUtilAsinhLargeArgument

Gradle alternative: copy the test into the matching module's src/test/ tree of the checkout and run ./gradlew -p lucene/<module> test --tests <class> -Ptests.seed=<seed> (add -Ptests.asserts=false for a run without assertions).

Formal models and scripts this finding's part of the hunt used: numeric-string-version-utils

Feedback

Open an issue in lucene-ai-bug-hunt. Please do not open issues or pull requests in apache/lucene about this finding unless a Lucene committer agrees it should be filed.

Snapshot of 2026-11-03. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.